Natural gas was generally not drilled from the ground, it was created from coal. So it was made locally, and a place was needed to store it, and that is what those structures were for.
They are not needed today, because there are cheaper ways to do it. We can store it underground, and also just increase the pressure in the line, to store more gas.
Tom Scott has a video on them that is interesting, and only about 2 minutes long. You will have to search for it on YT, as ELI5 doesn’t like links to videos.
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